1. Field of the Invention
The invention concerns a control process for a continuous skin pass and reduction operation for a metal strip in a skin pass mill comprising at least two successive roll stands.
2. Description of the Related Art
By skin pass and reduction operation is meant an operation conventionally called a "skin pass" or a second reduction rolling operation (called "DR"). For this type of operation one generally uses roll mills comprising at least two successive stands, of which only one, called the reduction stand, accomplishes the essential portion of the thickness reduction of the strip. In the case of a roll mill consisting of two stands, the reduction stand is the upstream stand or first stand, and in a roll mill consisting of three stands, the reduction stand is the second stand; the one in the middle of the installation.
A roll or skin-pass stand of this type of mill comprises two working rollers rotating in opposite directions, in the gap of which a metal strip can be reduced and/or cold-rolled. Depending on the squeezing force of these rollers, the thickness of the metal strip is reduced and its length is increased. For a given metal strip, the (length) extension rate achieved depends upon the squeezing force of the stand. In the case of a "skin-pass," the extension rate is low, while in the case of a "DR" operation, it can reach approximately 60%.
In these installations, the rotation speed of the rollers of two successive stands (called "stand speed" for short) must be precisely controlled in order to maintain a strip tension between these stands that is high enough to prevent the appearance of creases yet also low enough to prevent a risk of the strip breaking. Therefore between two successive stands, the stand speed differential depends on the extension rate of the strip.
For controlling these mills, one therefore generally determines the extension rate of the strip as it passes through the mill and the tension of the strip between the stands. Depending on the values measured, one acts on either the squeezing force of each stand or the speed of the rollers of each stand. To determine the extension rate of the strip in a stand, one generally measures the speed of the exiting strip and the speed of the re-entering strip. The extension rate is deduced from the difference between these two speed measurements. This type of process is described in British patent 794 290 where the speed measurements are made using speed.
In the case of a mill comprising only two stands, the article by Yuli Shimoyama et al., entitled "Kawaski continuous annealing line at Chiba"--IRON & STEEL ENGINEER, Vol. 69, No. 11, 1992, p. 35-41, describes three control processes on page 37, left-hand column, that are illustrated in FIG. 4 of that document:
In control according to mode 1, the extension rate is controlled by the inter-stand traction.
In control according to mode 2 (the references are to FIG. 1 hereinafter), one controls lengthening by the ratio between the speed of the upstream "S-block" 1' and that of the first downstream stand 2. Control (dead strip "ATL" system) of the traction between the "S-block" 1' and the first stand 2 is according to the speed and squeezing ratio of the first stand 2.
Control according to mode 3 is identical to that of mode 2, but the extension rate measurements are replaced by thickness measurements ("THG").
Mode 2 is schematized in the diagram in FIG. 1, where 1', 2 and 3 designate respectively the "S-block," the first stand and the second stand. Elements 4A and 4B are the device for measuring the extension rate.
In the case of a roll mill comprising three stands, the article by C. SILVY-LELIGOIS entitled "Extension control in dual reduction in Sollac Basse-Indre"--REVIEW of METALLURGY, Vol. 89, NO. 12, 1992, p. 1101-1109--describes another control strategy. There control of the inter-stand traction by the speed, and control of extension is via the squeezing of stand 2 and via the traction between stands 2 and 3.
This strategy is schematized hereinafter in FIG. 2, which reproduces FIG. 3 of the article cited. In this FIGS., 1, 2 and 3 designate respectively the first, second and third stand, and 4A and 4B designate the device for measuring the extension rate.
Finally, FIG. 3 shows the state of the art in the case of a mill comprising only two stands 2 and 3. There, control of the inter-stand traction by the speed, and control of extension is by squeezing and, optionally, inter-stand traction. This type of control is also described in FR 2 584 631 (MITSUBISHI).
Thus, as illustrated in FIGS. 1 to 3, to control this type of skin-pass mill, one makes a dual adjustment:
A--adjustment of the speeds of two successive stands according to the tension value of the strip between these stands, adapted to prevent breaking of the strip or the appearance of creases.
B--adjustment of the squeezing force of the reduction stand according to the difference between the strip extension rate provided by this stand and a predetermined extension rate set-point.
The transfer function of adjustment B is complex since the law of behavior linking force and extension is not at all linear. This law of behavior is similar to the standard law of behavior linking traction and extension, as illustrated in FIG. 4. According to this law, as the extension increases from zero, the traction begins by increasing sharply, then decreases slightly in irregular fashion before increasing slightly again and stabilizing. As the actions on the squeezing force (actuating element of adjustment B) have an impact on the tension value of the strip between two stands (measurement of adjustment A), the adjustment A will interact with the adjustment B.
Therefore it is important that the response time of adjustment A be sufficiently rapid with respect to that of adjustment B so that the changes in the squeezing force do not risk causing breaks in the strip or creases between two successive stands. Adjustment B is a "dead strip" type of adjustment.
This type of control process for skin-pass mills has numerous disadvantages for adjustment of the strip extension rate (adjustment B), including, among others, a response time that is too long, for it must remain longer than that of adjustment A; insufficient precision, more particularly due to the dead strip and the complexity (non linear) of the transfer function.